Many analog signal conditioning applications present unique challenges, but high-accuracy field applications can be especially difficult. The dynamic range of signals that must be supported may impose additional constraints.
One example problem encountered in a field application is that of electric service metering accuracy verification in the electric utilities industry, particularly in the area of electrical current-to-voltage conversion. This is due to the rapid changes in current amplitude that may span the entire conditioning range of the system. While computing energy by integrating power over an interval of time, any change in voltage or current not faithfully reproduced will produce significant error in the computed energy value. Therefore, a method is needed that is capable of precisely tracking a large range of current that may change amplitude quickly. In the case of current metering/monitoring, care must be taken in the design of signal conditioning circuitry placed on the output side of either a current transformer (CT) or a current source, to minimize errors and maximize the signal-to-noise ratio (SNR).
Some conventional current-to-voltage conversion devices employ different circuits that must be selected by switching between different circuitry according to the level of current involved. Conventional circuits may introduce too much burden on the secondary side of the current transformer causing a decrease in conditioning accuracy. Also, energy registration errors may be incurred while changing conditioning ranges as the switching process may produce transients or loss of data during the switching interval. Thus, a device to convert current with a low secondary burden is desired, and one that eliminates losses when the converted current changes amplitude rapidly over a large conditioning range.
Conventional techniques in common use for accurate, wide range conditioning of electrical current are often problematic in one or more aspects described herein. One such technique applies to conditioning of alternating electrical current using a current transformer (CT) coupled to an amplifier. This circuit is used to convert the CT current to a voltage signal that is an analogue of the CT current. The current-to-voltage amplifier is used in place of a discrete resistor to decrease the effective burden resistance applied to the output of the CT. Smaller burden impedance generally minimizes conditioning errors in the CT output current.
A current transformer (CT) is characterized by a primary winding having a small number of wire turns (which can be as small as one turn) around the transformer's magnetic core and a secondary winding on the same magnetic core having a large number of turns, possibly greater than 2000 turns. Typically, the magnetic core shape is toroidal. The current to be converted is a periodic alternating current connected to flow in the primary winding of the CT. This primary winding current induces a magnetic flux in the core that is an analogue of the primary current to be converted. In turn, the magnetic flux in the core induces a current in the secondary winding that consequently is also an analogue of the current to be converted except divided down by the ratio of secondary turns to primary turns. This step-down characteristic makes the CT particularly suited for the conditioning of large alternating currents.
A conditioning error intrinsic to use of CT devices for measuring electrical current is due to internal current division of the secondary current (is) between the magnetization and the leakage branches of the CT equivalent circuit as referred to the CT secondary. The magnetization branch impedance Zm is a parallel combination of a resistance Rc that accounts for the power losses in the core associated with the induced magnetic flux and an inductance Lm which accounts for the magnetization of the core. The leakage branch impedance Zl as described is a series combination of the leakage inductance Ll (due to imperfect coupling of the primary and secondary windings); the DC resistance Rw of the secondary winding wire; and the burden impedance Zb, which may be either resistive or inductive. Zb is the load placed on the CT output to receive and process the CT output current. The transfer function for the CT output current io due to internal current division with the secondary current inherent to the CT may be represented as:
      io    is    =      1          1      +              Zl        Zm            
Note that when Zm is much greater than Zl, the expression reduces to very close to unity indicating that virtually all of the secondary current (is) flows in the leakage branch Zl. Recall that Zl is a series impedance combination including the burden impedance Zb. Cores are chosen for CT current to current conversion devices that have large magnetic permeability to loss ratios to maximize both the inductance Lm and the parallel loss resistance Rc that make up the CT magnetization branch impedance Zm. This is one factor that contributes to a desirably smaller Zl/Zm ratio. Since the burden impedance Zb is series included in the leakage branch Zl, a larger Zb will produce a larger Zl and thereby will contribute to an undesirable larger decrease from unity of CT internal current division as shown in the above relationship. A more subtle effect of larger burden impedance Zb is due to the accompanying increase in voltage this produces across the magnetization branch impedance Zm. Larger voltage across Zm will increase the peak magnetic flux density in the core thereby increasing core losses. This will decrease the effective size of the core loss resistance Rc, also decreasing the magnetization impedance Zm. This can contribute to a reduction of the CT internal current division from unity.
More commonly, a resistor is coupled to the output of a CT to convert the CT output current to a voltage for subsequent use in downstream processing and/or control. These CT devices are the type typically used to convert very large alternating currents; i.e., hundreds or thousands of amperes expressed as root of the mean of the squared (RMS) current waveform magnitudes. The CT inaccuracies produced by these discrete burden resistors are therefore factored into the determination of the CT accuracy class.
Another common technique for increasing the range of current-to-voltage conversion is to use a “bypass” mechanism for the low range portion of the current-to-voltage conversion circuitry arrangement when the current level exceeds the upper limit of the low current range. When the converted current reaches a pre-determined threshold level, a bypass of the low range input is activated to reduce insertion impedance of the low range conditioning circuit input. Unfortunately, this also renders the low range output non-responsive to conditioning current so long as the current remains above the low current-to-voltage conversion range. Much work has been done in the design of these low range “bypass” electronic circuits to minimize unwanted conditioning error-inducing signal transients, long settling times, and changes of insertion impedance produced by the activation and imperfections of the bypass circuitry.
Use of a function to bypass the low range portion of a wide range (maybe several orders of magnitude) current-to-voltage conversion system can reduce the insertion impedance of the current-to-voltage conversion system. This is good for mitigating possible error due to an increase in impedance of the total load on a source of current to be converted. Ideally, a current-to-voltage conversion circuit should introduce a zero insertion or burden impedance load on a source of the current to be converted to avoid possibly changing the current to be converted. Wide range current-to-voltage conversion circuits may include a bypass function of the low current-to-voltage conversion input circuitry that is activated when the converted current exceeds the upper limit of the low range. This bypass circuit will continue to load the conditioning current source with another series shunt resistance. This series shunt resistance is required to provide an input to the accompanying high range current-to-voltage conversion circuitry of the system.
A current-to-voltage conversion requirement mentioned in the foregoing, and that may be associated with undesirable measurement errors, supports an associated electrical energy measuring (kW-hr). One application is directed to a calibration standard product used to verify the calibration of electrical power meters, in widespread use by power utilities, by a process referred to as registration. A standard measures electrical energy by integrating converted instantaneous electrical power (kW) continuously over an interval of time that can be several seconds to several minutes or longer. The standard computes electrical power from converted electrical voltage and converted flow of electrical current. The meter under test simultaneously measures power using precisely the same voltage and current so that the meter and the standard energy outputs can be directly compared or registered. When registering a meter using customer load voltage and current, typically the voltage will not change by more than a few percent during an energy conditioning interval, but the current flow is somewhat likely to change greatly during an energy conditioning. Therefore, a current-to-voltage conversion method is needed that responds instantaneously to rapid and large changes in converted current and has a conditioning range that can extend over several orders of magnitude while maintaining conditioning accuracy to better than 0.1% of reading continuously throughout the conditioning interval.